1. Field of the Invention
The present invention is a process for the very selective removal of 2-ring aromatics from straight run hydrocarbon distillate feed streams which contain low concentrations of 2-ring aromatics said process comprising contacting said hydrocarbon feed stream with one side of a polyester imide membrane under pervaporation conditions thereby permeating a high percentage of the 2-ring aromatics in the feed across the membrane resulting in the production of a permeate enriched in 2-ring aromatics and a retentate of severely reduced 2-ring aromatics content as compared to the hydrocarbon feed.
2. Description of the Related Art
The separation of aromatics from hydrocarbon streams comprising mixtures of aromatic and non-aromatic hydrocarbons using membranes is a process well documented in the literature.
U.S. Pat. No. 3,370,102 describes a general process for separating a feed into a permeate stream and a retentate stream and utilizes a sweep liquid to remove the permeate from the face of the membrane to thereby maintain the concentration gradient driving force. The process can be used to separate a wide variety of mixtures including various petroleum fractions, naphthas, oils, hydrocarbon mixtures. Expressly recited is the separation of aromatics from kerosene.
U.S. Pat. No. 2,958,656 teaches the separation of hydrocarbons by type, i.e., aromatics, unsaturated, saturated, by permeating a portion of the mixture through a non-porous cellulose ether membrane and removing permeate from the permeate side of the membrane using a sweep gas or liquid. Feeds include hydrocarbon mixtures, e.g., naphtha (including virgin naphtha, naphtha from thermal or catalytic cracking, etc.).
U.S. Pat. No. 2,930,754 teaches a method for separating hydrocarbons, e.g., aromatic and/or olefins from gasoline boiling range mixtures, by the selective permeation of the aromatic through certain non-porous cellulose ester membranes. The permeated hydrocarbons are continuously removed from the permeate zone using a sweep gas or liquid.
U.S. Pat. No. 4,115,465 teaches the use of polyurethane membranes to selectively separate aromatics from saturates via pervaporation.
Compared to distillation, membrane permeation can lead to considerable energy savings. A membrane can separate a mixture of aromatics and saturates, e.g., a heavy cat naphtha, into a highoctane, mainly aromatic permeate and a high-cetane, mainly saturated retentate. Both permeate and retentate are more valuable than the starting heavy cat naphtha.
Polyurea/urethane membranes and their use for the separation of aromatics from non-aromatics are the subject of U.S. Pat. No. 4,914,064. In that case the polyurea/urethane membrane is made from a polyurea/urethane polymer characterized by possessing a urea index of at least about 20% but less than 100%, an aromatic carbon content of at least about 15 mole percent, a functional group density of at least about 10 per 100 grams of polymer, and a C-O/NH ratio of less than about 8.0. The polyurea/urethane multi-block copolymer is produced by reacting dihydroxy or polyhydroxy compounds, such as polyethers or polyesters having molecular weights in the range of about 500 to 5,000 with aliphatic, alkylaromatic or aromatic diisocyanates to produce a prepolymer which is then chain extended using diamines, polyamines or amino alcohols. The membranes are used to separate aromatics from non-aromatics under perstraction or pervaporation conditions.
The use of polyurethane imide membranes for aromatics from non-aromatics separations is disclosed in U.S. Pat. No. 4,929,358. The polyurethane imide membrane is made from a polyurethane imide copolymer produced by end capping a polyol such as a dihydroxy or polyhydroxy compound (e.g., polyether or polyester) with a di or polyisocyanate to produce a prepolymer which is then chain extended by reaction of said prepolymer with a di or polyanhydride or with a di or polycarboxylic acid to produce a polyurethane/imide. The aromatic/-non-aromatic separation using said membrane is preferably conducted under perstraction or pervaporation conditions.
A polyester imide copolymer membrane and its use for the separation of aromatics from non-aromatics is the subject of U.S. Pat. No. 4,946,594. In that case the polyester imide is prepared by reacting polyester diol or polyol with a dianhydride to produce a prepolymer which is then chain extended preferably with a diisocyanate to produce the polyester imide.
U.S. Pat. No. 4,962,271 teaches the membrane separation under perstraction conditions of a distillate to produce a retentate rich in non-aromatics and alkyl-single ring aromatics and a permeate rich in multi-ring aromatics. The multi-ring aromatics recovered in the permeate are alkyl substituted and alkyl/hetero-atom substituted multi-ring aromatic hydrocarbons having less than 75 mole % aromatic carbon. The multi-ring aromatics are 2-,3-,4-ring and fused multi-ring aromatics.
U.S. Pat. No. 4,944,880 teaches polyester imide membranes and their use for the separation of aromatic hydrocarbons from feeds comprising mixtures of aromatic and non-aromatic hydrocarbons. The polyester imide membranes are described as being produced from a copolymer composition comprising a hard segment of polyimide and a soft segment of an oligomeric aliphatic polyester wherein the polyimide is derived from a dianhydride having between 8 and 20 carbon atoms and a diamine having between 2 and 30 carbon atoms and the oligomeric aliphatic polyester is a polyadipate, a polysuccinate, a polymalonate, a polyoxalate or a polyglutarate. The separation of aromatics from non-aromatics may be conducted under perstraction or pervaporation conditions. The hydrocarbon feed streams can be selected from heavy cat naphtha, intermediate cat naphtha, light aromatics content streams boiling in the C.sub.5 -150.degree. C. range, light cat cycle oil boiling in the 200.degree. to 345.degree. C. range as well as streams in chemical plants which contain recoverable quantities of benzene, toluene, xylene or other aromatics in combination with saturates.
These separations have involved the bulk separation of large amounts of aromatics from hydrocarbon streams which contained high concentrations of aromatics of various types. No one aromatic or aromatic type is enriched in the permeate to a very large degree relative to the other aromatics present in the permeate.
It would be extremely useful if trace or very low concentrations of specific aromatic components present in hydrocarbon streams could be selectively removed from such streams without resorting to exotic solvents in solvent extraction or complicated extractiondistillation processes. It would be especially attractive if such separations could be accomplished in a non-energy intensive manner such as membrane separation,